This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Appln. No. 2001 1010/01 filed in Switzerland on Jun. 1, 2001; the entire content of which is hereby incorporated by reference.
The present invention relates to a burner for a gas turbine or hot-gas generation for the combustion of liquid or gaseous fuel and to a method for operating it.
A principal problem which has to be solved within the framework of the development of industrial premixing burners for use in gas turbines or for hot-gas generation is the stabilization of the flame primarily in the part-load operating mode. Most industrial burners of this type utilize a swirl flow for generating a backflow zone on the burner axis. In these burners, flame stabilization takes place aerodynamically, that is to say without special flame holders. In this case, the backflow zones, which occur during the breakdown of the vortex, or the outer recirculation zones are utilized. Hot exhaust gases from these zones in this case ignite the fresh fuel/air mixture.
A burner according to the prior art, in which, for example, a backflow zone of this type is formed on the axis of the burner, is described in EP 0 210 462 A1. In the dual burner, specified there, for a gas turbine, the swirl body is formed from at least two double-curved metal plates acted upon by tangential air inflow, the plates being folded so as to be widened outward in the outflow direction. During outflow into the combustion chamber, a backflow zone at the downstream end of the inner cone is formed on the axis of the burner as a result of the increasing swirl coefficient in the flow direction. The geometry of the burner is in this case selected such that the vortex flow at the center has low swirl and axial velocity excess. The increase in the swirl coefficient in the axial direction then leads to the vortex backflow zone remaining in a stable position.
Further examples of what are known as double-cone burners are found in the prior art in EP 0 321 809 B1 and in EP 0 433 790 B1. In these burners with a conical shape opening in the flow direction, in which there are two part-cone bodies which are positioned one on the other and the center axes of which run, offset to one another, in the longitudinal direction, combustion air flows through the tangential inflow slots formed as a result of the offset into the interior of the burner. Simultaneously, during inflow through these slots, fuel is admixed with the combustion air, with the result that a conical fuel/combustion-air cone is formed and, again, a backflow zone in a stable position is formed in the region of the burner mouth.
In burners of this type, a power output reduction is achieved principally by a reduction in the fuel mass flow, with the air mass flow remaining approximately constant. That is to say, in other words, that, with a decreasing power output, the fuel/air mixture becomes increasingly leaner. However, since modern premixing burners are already operated near the lean extinguishing limit for the purpose of NOx minimization, other combustion concepts have to be developed for the part-load operating mode, in order to prevent extinguishing or an unstable behavior in the case of an increasingly leaner fuel/air mixture.
The prior art discloses, as combustion concepts for the part-load operating mode, for example, what is known as burner staging, in which individual burners are switched off in a specific manner, so that the remaining burners can be operated under full load. Particularly in the case of annular combustion chambers with a plurality of mutually offset burner rings having a different radius, this concept can be employed with a certain amount of success.
On the other hand, the transition from premixing combustion to diffusion-flame-like combustion is proposed, which, as is known, has a lower extinguishing limit in relation to the temperature. Consequently, a double operation of individual burners, which is employed according to the load, to be precise a premix-like and a diffusion-like operation, is proposed, in order to prevent extinguishing in the part-load mode. The problem with this, however, is that, on the one hand, it is complicated to design a burner for two different operating modes and, on the other hand, diffusion-like combustion usually cannot be carried out optimally in terms of emissions.
EP 0 866 267 A1 discloses the mixing of fresh air with recirculated smoke gas in the mirror-symmetrically tangentially arranged feed ducts of a double-cone burner in the case of atmospheric combustion. The combustion air enriched with the recirculated exhaust gas gives rise, for example, to better evaporation of the liquid fuel fed, via a central fuel nozzle, within the premixing zone induced by the length of the premixing burner. Although a lowering of pollutant emissions can consequently advantageously be achieved, nevertheless one disadvantage in a stabilization of the burner during the starting phase is that it is necessary to have a blow-off device which is connected operatively to the air plenum and by the use of which the admission pressure in the plenum is lowered, the air mass flow through the burner is reduced and consequently the air ratio is decreased.
The object of the invention is, therefore, to make available a burner for a gas turbine or hot-gas generation for the combustion of liquid or gaseous fuel, in which burner fuel is mixed with combustion air in a burner interior, is fed to a combustion chamber and is burnt in this combustion chamber, and a method for operating a burner of this type, which makes it possible to have a stable part-load operating mode.
As already mentioned above, double-cone burners from the prior art cannot achieve the abovementioned object, since, because operation is already lean in the full-load mode, in the part-load mode the flame becomes unstable or is even extinguished.
The present invention achieves the object by the provision of means which can stabilize the flame in the part-load mode.
The subject of the invention is consequently a burner of the abovementioned type, in which means are provided which make it possible to recirculate hot exhaust gas out of the combustion chamber into the burner interior for stabilization in the part-load mode.
The essence of the invention is, therefore, that the hot exhaust gases from the combustion chamber are used to stabilize the flow behavior in the burner interior and near the burner mouth, particularly in the part-load mode, that is to say during lean operation with reduced power output. Such recirculation of exhaust gases makes it possible to use burners of this type in machines (in particular, machines with variable inlet guide vane assemblies, VIGV) in a load range 30-100%.
According to a first preferred embodiment of the invention, the means are a recirculation line which, furthermore, picks up preferably hot exhaust gas on an axial combustion chamber wall near outer backflow zones present next to the burner mouth issuing into the combustion chamber and which feeds it to the burner interior in the region of a burner tip facing away from the combustion chamber. In such recirculation of the hot exhaust gases from a backflow zone, this recirculation takes place usually passively, that is to say the flow of hot exhaust gas into the burner interior does not have to be driven.
Another embodiment of the invention is distinguished in that the burner has at least one inner backflow zone. In a burner of this type, the result of the recirculation of the hot exhaust gases is that precisely this inner central backflow zone is stabilized on the axis of the burner by these hot exhaust gases.
In a further embodiment of the invention, the burner is a double-cone burner with at least two part-cone bodies positioned one on the other and having a conical shape opening toward the combustion chamber in the flow direction, the center axes of these part-cone bodies running, offset to one another in the longitudinal direction, in such a way that tangential inflow slots into the burner interior are formed over the length of the burner, through which inflow slots combustion air flows in, fuel being injected at the same time into the burner interior, so as to form a conical swirling fuel column and, subsequently, the mixture flows out, so as to form an inner backflow zone, into the combustion chamber and is burnt there. Particularly in the case of a double-cone burner of this type, the stabilization of the backflow zone on the burner axis can commence efficiently. In this case, the inner central backflow zone is stabilized particularly effectively when the hot exhaust gas is fed to the burner interior centrally in the vortex core, that is to say essentially on the burner axis, and, moreover, preferably as near as possible to the burner tip, that is to say at the point of the double-cone burner with the smallest diameter. The recirculation of the hot exhaust gases may in this case even take place actively in such a way that, in particular in the part-load mode, an inner backflow zone is completely or partially prevented.
According to a further embodiment of the invention, moreover, means are provided which make it possible to admix fuel with the hot recirculated exhaust gas. In combination with the increased temperature of the hot exhaust gases, this admixing of fuel leads to a selfigniting mixture being fed to the burner interior. Preferably, furthermore, fuel injection, exhaust-gas temperature and flow velocity are coordinated with one another in such a way that selfignition of the fuel takes place in the combustion chamber.
According to another preferred embodiment of the invention, not only fuel, but additionally also pilot air, is admixed with the recirculated hot exhaust-gas air. The admixing of the pilot air may in this case take place on the injection principle, that is to say in a way which drives the exhaust-gas air stream. By the additional introduction of pilot air into the exhaust-gas air duct, the burner can be actively regulated optimally in the part-load mode, using only a little additional air. To be precise, the usually cold pilot air may, on the one hand, be used for setting the temperature of the recirculated exhaust-gas air, but, on the other hand, the pilot air may also be utilized for increasing or lowering the exhaust-gas air stream, that is to say the flow velocity. Consequently, with the aid of the pilot air, selfignition, that is to say, in particular, the selfignition location of the mixture of hot exhaust gas and the fuel in or upstream of the burner interior in the combustion chamber, can be set exactly, that is to say optimized in terms of the influence exerted on the backflow zones.
The present invention relates, furthermore, to a method for operating a burner, such as is described above. Thus, in particular, exhaust gas recirculation is cut in and cut out as a function of the instantaneous power output stage of the burner, and, in particular, preferably the recirculation of hot exhaust gas is employed in the part-load mode. According to a preferred embodiment of the method mentioned, in this case the pilot-air stream is used for controlling the formation of the inner backflow zone or else also in order to block the recirculation of the exhaust-gas air, so that the swirl of the main airflow is sufficient to cause a breakdown of the vortex.
Further preferred embodiments of the burner and of the method are described in the dependent patent claims.